The Geology of Indonesia/The lesser Sunda Islands

The Lesser Sunda islands (LSI) are defined as a group of small islands situated between east of Java Island and Banda Islands, which are part of Western Banda Arc. Geologically, they constitute the inner outer arc built by Tertiary volcanic arc and some of the young volcanoes are still active. The arc is a south-facing island arc, which is the eastward continuation of the Sumatra-Java island arc system of Indonesia (Cardwell and Isack 1981). In terms of Plate Tectonic theory, the series of volcanoes built in the LSI is due to the subduction of Indo-Australian Oceanic crust beneath the LSI, and it is interpreted that the sources of melted magma from the subducted oceanic is about 165-190 Km depth (Hamilton 1979).

The oldest rock exposed in LSI is of Miocene age. Whereas to the western part of the islands (Sunda Arc) Mesozoic sequences are common (Foden and Varne 1981, Katili 1975). This indicates that the arc is relatively young tectonic product in the Indonesian Archipelago.

The systematic relationship between volcanoes, earthquake and active tectonics features at the region can give a better understanding of the present tectonics of the LSI. Although these phenomena are mainly considered due to the fact that the lithosphere of the Indian-Australian plate is being subducted beneath the Banda Arc, but the tectonic process are different from one place to other places along the subduction. These result in different type of volcanic composition and tectonic style along the arc.

Contents

The Lesser Sunda Islands (LSI) are defined as an area situated between eastern part of Java Island and Western part of Banda Arc, consisting of group of small islands and basins (Figure 1). Physiographically, It is bounded to the west by the Java Island, to the east by the Banda arc, to the north by the Flores Sea and to the south by Indian Ocean. Geologically, the Lesser Sunda Islands (LSI) is located in the center of Banda Arc, which were built by the young volcanoes forming geanticline. On the basis of Plate Tectonic Theory, the series of volcanoes built in the LSI is due to the subduction of Indo-Australian oceanic crust beneath the LSI, and it is interpreted that the sources of melted magma is about 165-200 km depth based on Hamilton’s tectonic map (1979).

In terms of Plate tectonic setting the Indonesian archipelago is situated in the triple junction of the three major plates, they are the Indo-Australian, the Eurasian and the Pacific Plates (Figure 2). The interaction of the three major plates creates a complex tectonic especially in the plate boundary that is situated in Eastern Indonesia. The LSI is mainly form as a result of the subduction of the Indo-Australian plate beneath the Sunda-Banda Arc during the upper Tertiary in which this subduction formed the inner volcanic arc of the LSI. However, there are differences in relation to the chemical analyses among the volcanic rocks in the LSI. The volcanic arc in the east Sunda region, which rests directly on oceanic crust and is bounded by ocean crust on both sides, has lavas with chemical characteristics that distinguish them from lavas in the western parts of the arc (Barber et al 1981). Hamilton (1979) suggested the inner ridge is formed of upper Cenozoic calc-alkalic rocks.

The volcanic rocks in the inner Banda Arc of the LSI in which the oldest known rocks are Early Miocene are found about 150 Km above the inclined zone of earthquake (Hamilton 1979, Audley-Charles 1981). The very active Benioff Zone was contoured by Hatherton and Dickinson (1989) and updated by Hamilton (1978) (Figure 3). Seismicity in the Java Sector extends to a maximum depth of about 600 Km. This indicates subduction of sub-ocean lithosphere belonging to the Australian/New Guinea Plate below the Banda Arc, and the cessation of volcanism in the Early Pliocene opposite Timor suggests collision of Timor with Alor and Wetar, after all the oceanic lithosphere had been consumed by subduction.

The size of the islands of this volcanic chain gradually is getting smaller eastward from Java through Bali, Lombok, Sumbawa, Flores, Wetar to Banda. This decreasing, which is most noticeable east of Wetar, may reflect the amount of ocean floor subducted, implying either that dip-slip motions have been more important westwards from Wetar and strike-slip motions increasingly important eastwards. Alternatively, it may be that the present volcanic arc east of Wetar is younger and perhaps that the original volcanic arc east of Wetar has been overridden by the Australian continental margin (Bowin et al. 1980)

Based on Plate Tectonic Theory, the LSI can be divided into four tectono-structural units consisting of from north to south (Figure 4); The back arc unit which is occupied by the Flores Sea; The inner arc unit which is built by a series of volcanic island consisting of Bali, Lombok, Sumbawa, Komodo, Rinca, Flores, Adonora, Solor, Lomblen, Pantar, Alor, Kambing and Wetar; The outer arc unit which is formed by non volcanic island of Dana, Raijua, Sawu, Roti, Semau and Timor; And the fore arc unit which is situated between the inner arc and outer arc and is part of deep basins consisting of Lombok and Savu basins.

In this paper, the author will mainly discuss the geology and geophysics of the inner arc and back-arc of the LSI. Whilst the rest will be discussed somewhere in this book by another author.

III. 1 BACK ARC REGION

III.1.1 Introduction

The back arc region of the LSI is mainly occupied by the Flores Sea which can be divided into three morphological units (van Bemmelen 1949); the NW Flores sea, The central Flores Basin, the east Flores Sea (see Figure 1). Bathymetric contours in the Flores Sea are oriented E-W (see Fig. 1). The most prominent phenomena is the symmetrical Flores Depression, where the water depth exceeds 5000 meters. Towards the volcanic arc, the continental shelf is narrow and very steep, suggesting structural control.

The NW Flores Sea is the broad and shallow platform connecting the south Arm of Sulawesi and Sunda Shelf with the water depth less than 1000 m. The Central Flores Basin has a triangular form with the top of triangular pointing to the direction of the Lampobatang volcano. Whilst the east Flores Sea comprise the ridges and interjacent trough, which connect the South Arm of Sulawesi with the submarine Batu Tara Ridge in the west of South Banda Basin.

III.1.2 Back-arc thrusting

In the Flores Sea, the phenomena of back-arc thrusting have attracted considerable attention and various hypotheses have been proposed to explain its initiation and driving mechanism. Silver et al. (1983) listed some of these hypotheses, including gravitational body forces as the sole mechanism, gravity spreading as a result of existing relief or injection of magma in the volcanic arc, low angle subduction resulting in back arc thrusting and collisional stress. Silver et al.(1983; 1986) considered back-arc thrusting to be a precursor of arc polarity reversal.

There are two major zones in the back-arc region of the LSI where back arc thrusting occurs (Silver et al. 1986; Prasetyo and Dwiyanto 1986); one is to the north of Wetar and Alor (Wetar Thrust), the other is to the north of Flores and Sumbawa islands (Flores Thrust). Hamilton (1979) proposed that these back arc thrusts indicated subduction polarity reversal due to the difficulty of subducting the buoyant continental margin of Australia, whereas Silver et al. (1983) related the distribution of back arc thrusts to the thickness of the forearc crust. Thick forearc crust, represented by Sumba Island and Timor Island, respectively is correlated with the formation of the Flores and Wetar thrusts.

III.1.3 stratigraphy and structure

Stratigraphy

Silver et al. (1986) divided the outer slope and trench sediments in the Flores Basin into five recognizable seismic stratigraphic units (Fig. 5), but not all units appear on all sections. The following divisions are based on their description. The lowest reflector (B) is irregular, hummocky, and considered to be acoustic basement. It was suggested that on Line 14, at least, this basement may consist of young (Pliocene ?) lava flows, consistent with the occurrence of volcanic products on the islands of the Bonerate Group to the north. Unit 1, which immediately overlies basement, is highly reflective and very variable in thickness. On Lines 8-12 this unit can be seen clearly and may thicken westward. Above Unit 1 is a poorly reflective Unit 2 which is inferred to have been deposited under pelagic or hemipelagic conditions. Unit 3 is well layered with variable thickness and is interpreted as a turbidite deposit. Above Unit 3 is another poorly bedded hemipelagic layer that generally forms the top of the slope sequence. This unit appears on all sections and is very thick in some places, although the average thickness is nearly constant at a few hundred meters. Unit 5 consists of trench turbidites. These are present on all profiles but vary greatly in thickness and lateral extent.

The stratigraphic framework in the western part of Flores Sea near the Bali Basin can be described in terms of four major units(Guntoro 1999), these being metamorphic basement, pre-rift sediments, syn-rift sediments and post rift sediments (Figure 6a and b). In some places diapiric intrusions reach up to the post rift sediments. The movement of diapirs was associated with Neogene compression, which resulted in the formation of thrust faults.

Structure

The structure of the western part of the Flores Sea as shown in Figure 6a and b is based on seismic refelection profile (Guntoro 1999). The interpretation indicates the presence of rift structure in the Palaeogene which later on experienced compressional tectonics in the Pliocene forming inverse structure. This seismic section can be described in terms of two major depressions (north and south), separated by the Lombok High. Structures can be recognized as due to folding, thrusting, block faulting, diapiric intrusion and igneous intrusion.

The older thrust faults, extending from basement through the pre-rift sediment units, were associated with Cretaceous compression which caused uplift of the region which was followed by erosion. Normal faults, extending from basement through the syn-rift sediments, were associated with the Paleocene-Eocene extension which produced horst and graben structures. Younger thrusts, extending from basement into the post-rift sediments, were associated with the reactivation of normal faults. The timing of inversion can be deduced from flexures observed in seismic sequences identified as post Middle Miocene to Pliocene.

Figure 7 shows seismic, bathymetric and gravity data obtained along the Mariana 09 line which crossed the Flores Sea from south to near Kabaena Island in the northeast, a total distance of about 400 km. The seismic profile shows that the region is dominated by series of down thrown basement blocks and magmatic activity. The Flores Ridge, the Flores Thrust, the Flores Basin, Selayar Ridge, Selayar Basin, Bone Ridge, Kabaena Basin and Kabaena Ridge can all be identified. The sediments accumulated in several sub basins separated by oceanic topographic highs. There are two principal types of reflectors. The first is characterised by rounded, conelike structures, irregular surfaces and high morphology, can be seen in the Selayar Ridge, Bone Ridge and Kabaena Ridge. The second reflector type is sub-parallel to horizontal. In the Kabaena and Selayar basins this reflector is interpreted as indicating flat-lying sediments but in the Flores Basin, where reflectors are smoother than in the Kabaena and Selayar basins, they probably mark basaltic lava flows with thin sedimentary cover.

Summary

From stratigraphic and structural point of views, it is clear that the back arc unit of the LSI have different history and evolution. The western part of the Flores Sea show similar to the basin in the Java Sea in which the history started since Mesozoikum. In contrast, the central Flores Sea indicates young basin of Pliocene age.

III.2 INNER ARC REGION

III.2.1 Introduction

The inner arc region of the LSI is situated on a geanticlinal ridge (van Bemmelen 1949), which has a width of about 100 km in its western end, gradually diminishing eastward to about 40 km. It consists of a row of young volcanic islands, 1400 km long, connecting the volcanic inner arc of the Banda system with the volcanic Java-Sumatra arc, they are of Bali, Lombok, Sumbawa, Komodo, Rinca, Flores, Adonora, Solor, Lomblen, Pantar, Alor, Kambing and Wetar; Van Bemmelen (1949) divided this volcanic inner row into an eastern and western part. The eastern part (from Romang to Komodo) represents the volcanic inner arc of the Timor orogene. Whereas the western part (From Sumbawa to Bali) is more closely related with the Java sector of the Sunda Mountain System.

III.2.2 Geology of the LSI

The geology and tectonic setting of the Lombok and Sumbawa are descibed below (qf. Foden and Varne 1981b). The islands of Lombok and Sumbawa lie in the central portion of the Sunda Arc. The oldest exposed rocks are Miocene, suggesting that subduction and volcanism began considerably later than in Java and Sumatra to the west, where there are abundant volcanic and intrusive rocks of Late Mesozoic age. The islands are located on the eastern edge of the Sunda shelf, in a zone where crustal thickness is apparently rapidly diminishing, from west to east (Curray et al, 1977). The seismic velocity structure of the crust in this region is transitional between typical oceanic and continental profiles and the Moho appears to lie at about 20 km (Curray et al. 1977). These factors tend to suggest that there has been limited opportunity for crustal contamination of magmas erupted on the islands of Lombok and Sumbawa. In addition, these islands lie to the west of those parts of the eastern-most Sunda and west Banda arcs where collision with the Australian continental plate is apparently progressing.

The volcanoes considered are located between 165 and 190 km above the benioff Zone (Hamilton 1979) (see Fig. 3). Rinjani, Tambora and Sangeang Api Volcanoes are still active, while Sangenges and Soromundi are eroded cones of Quaternary age (Sudrajat 1975). There is a marked offset in the line of active volcanoes between the most easterly Sumbawa Volcano (Sangeang Api) and the line of active volcano in the Flores. This suggests that a major transcurrent fault cut across the arc between Sumbawa Island and Flores. Audley-Charles (1975) considered that this feature represented a major tectonic discontinuity between the east and west Sunda Arcs (the Sumba Fracture). Further, Hedervari (1978) and Ritsema (1954) found a market absence of shallow and intermediate earthquake activity in the region to the south of Lombok and Sumbawa, a feature they interpreted to represent a marked break in the Sunda Arc Zone.

Faulting and folding caused strong deformation in the eastern part of Lombok Basin and is characterized by block faulting, shale diapirs and mud volcano (Prasetyo 1992). Abbot and Chamalaun (1978) suggested that Wetar formed island as part of island arc by the Late Miocene and great uplift occurred during Plio-Pleistocene.

III.2.3 Stratigraphy

The stratigraphy of the inner arc region of the LSI can be seen in Tables 1 and 2 (Suwarno and Noya 1985). The following stratigraphy description is quoted from these authors. The oldest rock exposed is Early Miocene volcanic rocks consisting of andesitic-basaltic breccia volcanic unit deposited in the marine environment. This unit is interfingering with tuffaceous sandstone unit (occur in Ruteng, Ende, Lomblen and Sumbawa islands) and limestone unit (occur in Komodo and Sumbawa), both units are also interfingering. Conformably, but in some other areas are unconformable, overlying the Early Miocene volcanic rocks is the reef unit (occur in Bali, Lombok, Sumbawa, Komodo, Ruteng) and lava dacitic unit (occur in Lombok, Komodo, Ruteng, Alor & Wetar Barat). Reef unit and lava dacitic units are interfingering with upper part of tufaceous sandstone (in Bali). Late Middle Miocene basaltic-granitic dykes intruded all rock units above.

In the eastern part of LSI, some dioritic - granodioritic intrusion rocks are part of andesitic-basaltic lava unit and Naumantang Formation. Toward the western part (Bali), equivalent with andesitic-basaltic tuff unit, coral reef unit and andesitic-basaltic lava unit is Selatan Formation (Msl) consisting of limestone covered conformably Ulakan Formation (Mu). It is suggested that conformably above the Selatan Formation is Prapatagung Formation (Ppa) and Pulaki volcanic rocks (Pp). Above Prapatagung Formation and Pulaki volcanic rocks are conformably overlain by Asah Formation (Pa) consisting of volcanic rocks and locally calcareous.

Covered unconformably most of the area is the old volcanic products (Qtv, Qot, and Qv) consisting of lava, breccia, agglomerate and andesitic sandy tuff of Plio-Pleistocene age.

The volcanic activity in the LSI continues up to the present time. The result can be seen as volcanic cones built by andesitic-basaltic rocks (qhv, Qyt, A, B, P, Qbb, Qvb, Qvc, Qvd, Qve). Those volcanoes grow well in land as well as on offshore.

The rocks which are considered as young products are coral reef (Ql), Terraces (Qt, Qct, Qalk), and alluvium deposits and beach (kW/qAL). All those rocks are deposited unconformably above the surrounding rocks.

Volcanic activity with strong explosion can be seen in Bali and Lombok with the result of caldera such as Buyan-Bratan Caldera, Batur Caldera, Rinjani Caldera. Th occurrence of batur Caldera is approximately 22.000+/-1500 years.

III.2.3 GEOLOGICAL HISTORY

Based on stratigraphical and structural data above, Suwarno and Noya (1985) proposed the geological history of the inner arc of the LSI. The geological history started in Early Miocene when the area constitutes a basin. The first deposition was clastic sediments forming tufaceous sandstone and limestone which were deposited in the marine environment with the depth of about 20-100 m (neritic). Before these clastic sediments were deposited, the area is influenced by marine volcanic activity forming andesitic-basaltic volcanics which were called andesitic-basaltic breccia volcanics. This volcanic activity indicates the subduction resulting volcanic arc in the LSI initially occurred in pre-Miocene and at least in Oligo-Miocene (Katili 1975).

In the mid Miocene, the andesitic-basaltic volcanic activity decreased, but it was replaced by the appearance of dacitic -rhyolitic composition material. They are represented by dacitic lava unit, upper part of tuffaceous sandstone unit, reef limestone unit and upper part of Mulakan Formation (Mu).

In Middle Miocene, locally in Sumbawa and Komodo there was tectonic activity indicated by unconformity between andesitic-basaltic volcanic breccia and reef limestone unit and dacitic lava unit.

In late Middle Miocene to early Upper Miocene, there were tectonic activity causing uplifting, faulting, and folding trending in NE-SW to NNW-SSE and also was associated with magmatic activity of various compositions. From Lombok up to Wetar, this event caused an obvious gap, but in Bali, this event is not clear. In Bali, marine sedimentary deposition continues up to Pliocene and even Quaternary producing Surga Formation, Selatan Formation, Prapatagung Formation and Pulaki volcanic rocks. from this fact, it is suggested that between Lombok and Bali, it was cut off by a fault trending in N-S with upthrown in the Lombok and downthrown in Bali.

Post early Upper Miocene, all area from Sumbawa to Wetar experienced subsidence. In contrast Lombok was part of a high separated basin in the eastern part (Sumbawa to Wetar) and in the western part (Bali), so it is suggested that Lombok was a high. In the eastern basin, there were deposition of tuff, tuffaceous sandstone and limestone, which are grouped in the andesitic-basaltic tuff unit and reef limestone unit with neritic-bathyal environment. Whilst in Wetar and Bali occurred marine volcanic activity producing lava, breccia, agglomerate and tuff in basaltic-andestic and dacitic composition with calcareous intercalation. These rocks are grouped into andesitic-basaltic lava unit and Naumantang formation. This deposition activity lasted up to Early Pliocene whereas in general volcanic activity on Early Pliocene wanned and produced tuff only.

In Plio-Pleistocene, the basins in Sumbawa-Wetar were uplifted with the formation of moderate-strong folding on late Pliocene or early Pleistocene. In that time, volcanic activity was increasing again producing andesitic-basaltic volcanic rocks (Qtv, Qot) and locally produced tephrite-lesit (Qv ls/le) like in Sumbawa.

In Late Pliocene to Pleistocene, Bali was also uplifted followed by volcanic activity producing non-marine andesitic-basaltic volcanic rocks (Qd, Qba, Qsv). This also occurred in Lombok (v1, v2, v3) and in Sumbawa-Wetar (upper part Qtv and Qvb, Qvs, Qvsa, Qvl, Qvm, Qvyo, Qv ls/le, Qvd dan Qvg). Locally in Bali was also deposited conglomerate, sandstone, and coral reef of Palasari Formation (Qp).

In Late Pleistocene, all area was uplifted and was followed by faulting and folding. Miocene to Pliocene rock units show faulting trending in NE-SW to NW-SE.

Late Pleistocene or Early Holocene volcanic activity lasts up to present day marked by the presence of 17 active volcanoes (Figure 3). This fact cannot be separated by the movement of Indo-Australian oceanic plate. Volcanoes in Bali-Sumbawa occupy northern part of the arc, whilst in Flores occupy the southern part of the arc. This volcanic activity produces andesitic-basaltic volcanic rocks which are grouped into young volcanic rocks (Qhv, Qyt, Qpb, Qvb, Qvc, Qvd, Qve, A, B, P) and cover some of the older rocks.

The presence of beach and river terraces (Qt and Qct), uplifted lacustrine deposits (Qalk) and coral reef (Ql), indicate that this area is still uplifting up to now. For Bali, the Quaternary uplifting produced Palasari Formation. The uplifting movement maybe still continue and cause the tilting of Bali island in which the northern part is steeper than the southern part.

Frequent earthquake shocks and active volcanoes in the LSI indicate to the active tectonic process, which are currently in progress in response to the continued movement of the Indo-Australian Plate beneath the arc. However toward the eastern of the subduction, in Alor and Wetar, the volcanoes is no longer active since the Pleistocene and it is interpreted due to the cease of subduction as the collision of the northern margin of Australian beneath the arc occurred.

Many authors assumed that the Sunda arc and Banda arcs are continues. However, some other authors suggested as discontinues. Some hypothesis have been put forward to support this opinion as explained below. Cardwell and Isack (1978) show no evidence of a major discontinuity in the seismic records. However; Audley-Charles (1975) proposed the Sumba fracture between Sumbawa and Flores as the structural discontinuity of the two arcs. Nishimura et al. (1981) proposed that a major tectonic discontinuity separates Eastern Indonesia from Western Indonesia between Sumbawa and Flores, this view being based on investigations of the differences in geophysical, geochemical and submarine morphological features. The regional Bouguer gravity anomaly patterns change considerably in the area between Sumbawa and Flores (Chamalaun et al. 1976). East of Flores Island, there are east-west gravity anomalies along the outer Banda Arc with high positive values in the north. West of Sumbawa Island there are also east-west gravity anomalies associated with the Java Trench system, but in the field decreases from high positive values to the south to low or negative values to the north. Between these two regions of opposite gradient is a region in which contour lines trend north-south. In geochemical studies, there are differences in the chemical characters of the Quaternary volcanic rocks of Lombok, Sumbawa and Bali on the one side and Flores on the other side. Discontinuity in the distribution of earthquake shocks beneath the Sunda Arc indicate that the underthrusting of the Indian ocean plate is actively in progress in this segment of the arc. Whist, shallow shocks are absent in the western segment of the Banda arc, indicating that to the east of Sumba underthrusting has ceased. Discontinuity in the trend of the volcanic arc, with volcanoes on eastern side of the discontinuity displaced southward with respect to those on the west. This offset occurred along an extension of the Palu-Koro transcurrent fault recognized in Sulawesi.

IV.2 Volcanic composition

The Sunda Arc is complicated by the change in the converging Indian Ocean Australian Plate, which is oceanic west of Timor and to the east is of 40 km thick continental crust, depressed by 3 km at the axis of the Timor trough (Bowin et al. 1980). The presence of active volcanoes from Bali to Flores indicate subduction related subduction. However, toward Alor and Wetar the volcanism is no longer active. This suggests that the geometry of subduction is not homogeneous along the arc. Volcanism in the islands of Alor and Wetar in the inner Banda Arc ceased in Early Pliocene time, it is due to the Australian continental margin had collided with this part of arc (Audley –Charles 1980). Hutchison (1981) found that the chemical characteristics of volcanoes around the arc systems could be correlated directly with their tectonic environment. Factors which influence the composition of the lavas included depth to the underlying Benioff zone, and whether the volcanoes are constructed on continental or oceanic crust. Petrography and geochemistry of the Quaternary-recent volcanoes of Lombok and Sumbawa arc is unusual in which the volcanic arc rests directly on oceanic crust (Foden and Varne 1981b). The oceanic crust occurs on both sides of the arc. The depth to the underlying Benioff Zone beneath the volcanoes is 165 km - 190 km. The volcanic rocks vary in composition from calk-alcaline to highly potassic, but there is no correlation between their composition and depth to the underlying Benioff Zone. Since there is no continental material in the surrounding of the arc, the highly potassic potassic leucitites cannot be attributed to contamination by continental material. He concluded that variations in the compositions of the lavas can only be attributed to inhomogenities in the source region.

In Lombok, Rinjani Volcano lies approximately 300 km north of the Java trench and is situated about 170 km above the active north dipping Benioff seismic zone (Hamilton 1979). Foden and Varne (1981) based on the composition of andesites which have very low Ni concentrations and low Mg/Mg+Fe, he suggested that the Rinjani suite is of mantle origin, but that all the andesites and dacites as well as many of the basalts have probably been modified by fractional crystallization processes. In brief, Foden and Varne concluded that the Rinjani calc-alkaline suite, which in many respects is typical of many suites erupted by circum-pacific volcanoes, probably originated by partial melting of the peridotite mantle-wedge overlying the active Benioff Zone beneath Lombok Island.

The LSI and its surrounding area is considered as an area of high seismicity, and several large earthquakes associated with tsunami have been recorded in this area. Figure 3 show the distribution of earthquake in this area from 1900 to 1988 with the magnitude varying from 3 to 7 Richter scale ((Ismanto and Prajuto 1989). Figure 2 also shows the depth and magnitude of the earthquake in north-south direction with the depth of epicenter reaching 550 km but other reports mentioned the depth up to 720 Km. The distribution of the earthquake indicate that the Australian Plate has been subducting to the north at an inclination of about 63 degree, and also the subduction of the oceanic crust in the Flores Sea to the south due to the subduction reversal polarity (See Figure 1 from Ismanto and Prajuto, 1989).

1. The LSI can be divided into four structural tectonic units, they are; The back-arc unit, the inner arc unit, the foe arc unit, and the ouetr arc unit. Each unit has different stratigraphic and structural styles. 2. From the information of Well JS 25-1, basement complexes are present in the Central Lombok Block. These are suggested to be part of the Late Cretaceous accretionary complexes formed at the southeastern margin of the Sundaland. Whilst, the basement in the Flores Sea may consist of young (Pliocene) lava flows (Silver et al. 1986), and thin sedimentary covers. 3. The western part of Flores Sea has experienced three major tectonic events, these being Cretaceous crustal compression, Palaeogene crustal extension and Neogene inversion. The changes between these structural regimes, indicated by distinct basin geometries, styles of faulting and structural deformation since the Cretaceous, imply changes in the intraplate stresses or plate boundaries and movements which can be linked to the evolution of subduction and collision around the SE Sunda Shelf. 4. The presence of back-thrust in the Flores Sea associated with high seismicity in this area of about 30 km depth indicates the initial stage of arc reversal polarity with the subduction of oceanic crust in the Flores Sea to the south toward the LSI. This has been interpreted due to the choked subduction of the Indo-Australian plate in the south towards the LSI. 5. Based on the similarity of lithology, depositional environment and age, it is interpreted that the area in the LSI is a basin during Early Miocene - Late Miocene. After the Late Miocene, Bali Island became a separate basin to the basin in the eastern part and were separated by a ridge in the Lombok Island. 6. The seismic velocity structure of the crust in this region is transitional between typical oceanic and continental profiles and the Moho appears to lie at about 20 Km (Curray et al. 1977).